[Production Method for Vapor-Phase Epitaxial Film]
Vapor-phase epitaxial growth means that, when a deposited film is grown on a single-crystal substrate in a vacuum atmosphere, the growth proceeds while maintaining a given relationship between the respective crystal orientations of the substrate and the deposited film under the influence of the orderly atomic arrangement of the crystal of the substrate. A typical thin film growth technology based on the epitaxial growth includes a sputtering process, a chemical vapor deposition (CVD) process, a molecular beam epitaxy (MBE) process and a pulsed laser deposition (PLD) process.
Currently, the sputtering process using non-equilibrium evaporation is most widely used and industrially applied. A sputtering process for use as a film forming method utilizes a so-called sputtering-phenomenon discovered by W. Grove in 1852. The sputtering phenomenon means that high-kinetic energy particles emitted onto the surface of a target (ions or atoms neutralized by electrons around the target surface) give the kinetic momentum of the particles to constitutive atoms of the target through an elastic collision therebetween, and the recoil atoms are finally released from the target surface after repeatedly colliding with adjacent atoms. The film forming method based on the sputtering process is intended to deposit the released constitutive atoms of the target on a substrate so as to form a thin film.
The chemical vapor deposition (CVD) process is intended to grow a thin film having a desired composition on a substrate by using chloride or organometal compound as a source material, and inducing the chemical reaction between source gases on the substrate. The CVD process having excellent compatibility to mass production is widely used to produce practical materials for semiconductor lasers and others.
The molecular beam epitaxy (MBE) process is a thin-film crystal growth technology named in 1968 by J. R. Arthur who was with Bell Laboratory at the time, and developed for compound semiconductors, primarily GaAs. The MBE process can be considered as an improved or progressive type of a vacuum deposition process.
Specifically, the MBE process is intended to perform an epitaxial crystal growth under the conditions that the flow of neutral molecules (or atoms) serving as raw materials of a crystal to be grown, or the intensity of a molecular beam (atomic beam), is accurately controlled in ultra-high vacuum, and the neutral molecules (or atoms) are emitted onto an accurately temperature-controlled substrate. The MBE process can be applied to a wide range of materials including compound semiconductors, elementary semiconductors such as Si, and various metal or oxide superconductors. The difference from the conventional vacuum deposition process is in that (i) the vacuum degree in a growth chamber is set at 10−7˜10−8 Pa or less, (ii) the crystal growth is performed while stably maintaining the cleanness of the crystal surface at the atomic scale, and (iii) the intensity of the molecular beam is accurately controlled.
The research and development of the pulsed laser deposition (PLD) process was started soon after the first success in ruby laser oscillation in 1960. From the last half of the 1980s, the pulsed laser deposition (PLD) process using an excimer laser has been actually applied to a production method for a single-crystal thin film of high-temperature superconducting material, dielectric material or organic polymer
Laser ablation means a process of etching the surface of a solid utilizing a phenomenon that when the intensity of laser light emitted onto a solid is increased up to a given threshold value or more, the laser light is converted into electronic, thermal, photochemical and mechanical energies on the solid surface, and consequently various kinds of fragments (neutral atoms, molecules, positive/negative ions, radicals, clusters, etc.) are explosively released from the solid surface.
The PLD process utilizes the principle of the laser ablation to produce a single-crystal thin film on a substrate under an ultra-high vacuum atmosphere set at a vacuum degree of 10−7˜10−8 Pa or less in a growth chamber, or under a sufficiently controlled atmosphere based on the flow of reactive gas. While the PLD process is a non-equilibrium process as with the sputtering process, it is different from the sputtering process in that the difference between the respective compositions of a target and an obtained film is small, and single-crystal films of various material can be produced by simply replacing a target. In recent years, the PLD process is actively used, particularly, in the field of research on exploration of substances, by utilizing the above features.